The basic concept of electrostatic spinning (or electrospinning) a polymer to form extremely small diameter fibers was first patented by Anton Formhals (U.S. Pat. No. 1,975,504). Electrostatically spun fibers and nonwoven webs formed therefrom have traditionally found use in filtration applications, but have begun to gain attention in other industries, including in nonwoven textile applications as barrier fabrics, wipes, medical and pharmaceutical uses, and the like.
Electrospinning is a process by which electrostatic polymer fibers with micron to nanometer size diameters can be deposited on a substrate. Such fibers have a high surface area to volume ratio, which can improve the structural and functional properties of the substrate. Typically, a jet of polymer solution is driven from a highly positive charged metallic needle to the substrate which is typically grounded. Sessile and pendant droplets of polymer solutions may then acquire stable shapes when they are electrically charged by applying an electrical potential difference between the droplet and a flat plate. These stable shapes result only from equilibrium of the electric forces and surface tension in the cases of inviscid, Newtonian, and viscoelastic liquids. In liquids with a nonrelaxing elastic force, that force also affects the shapes. When a critical potential has been reached and any further increase will destroy the equilibrium, the liquid body acquires a conical shape referred to as the Taylor cone.
Naturally derived as well as synthetic polymers like collagen, gelatin, chitosan, poly (lactic acid) (PLA), poly(glycolic acid) (PGA), and poly(lactide-co-glycolide) (PLGA) have been used for electrospinning. In addition to the chemical structure of the polymer, many parameters such as solution properties (e.g., viscosity, conductivity, surface tension, polymer molecular weight, dipole moment, and dielectric constant), process variables (e.g., flow rate, electric field strength, distance between the needle and collector, needle tip design, and collector geometry), and ambient conditions (e.g., temperature, humidity, and air velocity) can be manipulated to produce fibers with desired composition, shape, size, and thickness. Polymer solution viscosity and collector geometry are important factors determining the size and morphology of electrospun fibers. Below a critical solution viscosity, the accelerating jet from the tip of the capillary breaks into droplets as a result of surface tension. Above a critical viscosity, the repulsive force resulting from the induced charge distribution on the droplet overcomes the surface tension, the accelerating jet does not break up, and results in collection of fibers on the grounded target. Although the jet of fiber divides into many branches on its surface after the jet leaves the tip of the needle (Yarin, K Yarin, A. L., W. Kataphinan and D. H. Reneker (2005). “Branching in electrospinning of nanofibers.” Journal of Applied Physics 98(6):-ataphinan et al. 2005). If not controlled, the branches of the fibers create a non-uniform deposition on the substrate. An objective of this invention is to enable control of deposition of branches of the fibers to provide uniform distribution of the fiber on a substrate.
Many engineering applications require uniform distribution of the fiber on the substrate. For example, one of the most important cell morphologies associated with tissue engineering is elongated unidirectional cell alignment. Many tissues such as nerve, skeletal and cardiac muscle, tendon, ligament, and blood vessels contain cells oriented in a highly aligned arrangement, thus it is desirable that scaffolds designed for these tissue types are able to induce aligned cell arrangements. It is well documented that cells adopt a linear orientation on aligned substrates such as grooves and fibers. Aligned nanofiber arrays can be fabricated using the electrospinning method [Li D, Xia Y. Electrospinning of nanofibers: reinventing the wheel? Adv Mater. 2004; 16:1151-1170] and many studies have shown that cells align with the direction of the fibers in these scaffolds.
In addition to the influence on fiber arrangement, cell alignment can have positive effects on cell growth within tissue engineering scaffolds. Myotubes formed on aligned nanofiber scaffolds were more than twice the length of myotubes grown on randomly oriented fibers (p<0.05) and neurites extending from DRG explants on highly aligned scaffolds were 16 and 20% longer than those grown on intermediate and randomly aligned scaffolds respectively [Choi J S, Lee S J, Christ G J, Atala A, Yoo J J. The influence of electrospun aligned poly(epsilon-caprolactone)/collagen nanofiber meshes on the formation of self-aligned skeletal muscle myotubes. Biomaterials. 2008 July; 29(19):2899-906].
Growth of electrical bending instability (also known as whipping instability) and further elongation of the jet may be accompanied with the jet branching and/or splitting. Branching of the jet of polymer during the electrospin process has been observed for many polymers, for example, polycaprolactone (PCL)(Yarin, Kataphinan et al. 2005), polyethylence oxide (Reneker, D. H., A. L. Yarin, H. Fong and S. Koombhongse (2000) “Bending instability of electrically charged liquid jets of polymer solutions in electrospinning” Journal of Applied physics 87(9): 4531-4547). Such branching produces non-uniform deposition of fiber on the collector during the electrospin process. A method and apparatus to separate out a continuous single thread of fiber from many fiber branches has not been solved. A method is needed by which uniformly distributed single thread fiber can be deposited on a substrate during electrospinning processes for various engineering applications requiring uniform, controlled fiber deposition on a substrate, including enabling elongated unidirectional cell alignment.